As known, according to a widespread technique, labels are attached on the external surfaces of respective vessels or articles by means of glue previously applied on the back surfaces of the labels themselves.
In particular, labels are cut from a web at appropriate lengths and then advanced by a transfer drum towards an application station, at which labels are applied on the respective vessels or articles. Prior to reaching the application station, each label receives a layer of melted glue on its back surface by a gluing system including a rotatable glue roller cooperating tangentially with the transfer drum; in practice, as it is advanced by the transfer drum, each label contacts, on the opposite side thereof with respect to the transfer drum, the glue roller.
The gluing system further comprises:                a tank set to a given hot temperature, such as 140-150° C., suitable to maintain the glue in a melted state; and        a glue delivering line connecting an outlet section of the tank to the glue roller and adapted to continuously feed melted glue to the outer surface of the glue roller itself.        
A glue scraper is arranged at the periphery of the glue roller and at a certain radial distance from the lateral surface thereof; the glue scraper removes the excess of glue and smoothes the surface of the glue layer applied onto the lateral surface of the glue roller.
The excess glue is recovered in the tank to be then recirculated to the glue roller together with the fresh glue present in the tank itself.
The described gluing system requires a bulky tank containing a large mass of glue maintained at high temperature for many days; in these conditions, the properties of glue degrade very quickly.
In addition, a large mass of glue is continuously recirculated between the tank and the glue roller; during such recirculation, glue is exposed to atmospheric agents, which accelerate degradation of glue properties after a few cycles.
Another factor that tends to deteriorate the adhesive properties of the glue is the continuous mixing of the fresh glue with the older recirculated glue with consequent contamination of the fresh glue. Plus, due to this continuous mixing, some fractions of the glue may be recirculated for long times prior to being applied on the labels, with consequent very low adhesion performances.
Furthermore, the known glue feeding systems require high costs for manufacture as well as for maintenance.